1. Field
The present invention relates to photonic devices and more particularly to an apparatus and method for determining the centroid wavelength of a light beam.
2. Background
There is increasing demand for tunable lasers for fiber optic networks. Dense wavelength division multiplexing (DWDM) fiber optic systems encode separate data streams in a single optical fiber by assigning each data stream to a specific channel of wavelength or frequency. Present channel separations are 50 GHz (approximately 0.4 nm) such that, for example, the International Telecommunication Union (ITU) allocated C band from 192.1THz to 196.1 THz can support 81 channels.
DWDM systems have been largely based on distributed feedback (DFB) lasers. While DFB lasers have typically operated at a single wavelength or channel, recent technological developments have enabled narrowly tunable DFB lasers to address a few adjacent channels and widely tunable lasers have been developed that permit addressing many channels, for example all 81 of the C band 50 GHz channels or more.
One difficulty in exploiting laser tunability is in guaranteeing wavelength accuracy. One aspect of wavelength accuracy is the degree to which the lasing wavelength corresponds to one of the pre-defined channels. For example, the aging of the laser device or changes in the environment can cause a laser to drift in frequency, resulting in sub-optimal performance in the fiber optic network. Components known as wavelength lockers have been employed to combat this wavelength drift. Wavelength lockers are typically based on a passing a portion of the beam under test through reference etalon, which provides a transmission that is periodic in wavelength, and detecting the transmitted power with a detector. The wavelength locker can provide information regarding the accuracy of the test beam wavelength with respect to a spectrally periodic array of transmission peaks, and this information can be used to maintain the laser wavelength despite time dependent drifts.
A wavelength locker of the sort noted above is degenerate in etalon mode number. That is, there is an array of possible wavelengths that can give rise to the same signal from the wavelength locker. This poses a substantial problem for tunable lasers. Typically, a set of operating parameters is determined during a factory tuning calibration, and these parameters correspond to operation at the pre-defined operating channels. Unfortunately, the same factors that lead to wavelength drift can cause a drift in the parameter set needed to realize the pre-defined operating channels. In this case, a user command to tune the laser to a different channel will result in an inaccurate wavelength. A servo system based on a periodic wavelength locker will be able to correct wavelength inaccuracy only within a certain spectral capture range. For example, if the drift in the parameter set results in tuning the laser incorrectly by more than a half a channel, a servo system based on a wavelength locker will typically cause the laser wavelength to be updated to lock to the nearest pre-defined channel in the periodic array defined by the wavelength locker. This channel will by definition then be different than the one intended by the user. This error in absolute channel number will not be detected by this wavelength locker based servo system, and will result incorrect routing of the data.
Another issue facing laser manufacturers is packaging of photonic devices, as users demand additional functionality be integrated within industry defined form factors.